This invention relates to formation of electrical contacts to semiconductor devices.
Tape automated bonding (TAB) is one method of forming electrical connections to contact pads on a semiconductor device or circuit. The technique basically involves formation of conductive leads on a tape backing and then bonding the leads to the contact pads on the semiconductor by means of spheres or bumps initially attached to either the pads or the tape leads. The spheres or bumps, hereinafter also referred to as contact portions, act as buffers to absorb the impact of bonding the leads to the pads. The opposite ends of the leads can then be coupled to external circuitry.
TAB provides many advantages over standard techniques, such as wire bonding to lead frames, used in semiconductor fabrication. For example, since TAB leads can usually be made thicker than wire bonded interconnections, the technique generally produces less noise in high frequency applications. Further, since the TAB leads can also be made with a higher density than wire bonded interconnections, TAB is useful in devices requiring a high density of electrical connection to the semiconductor chip due to either a great number of required connections or reduced pad and chip sizes.
In spite of such advantages, TAB has not been employed widely in MOS devices or circuits. This is primarily due to the fact that it has been difficult to form a strong bond between the contact portions and the aluminum pad metallization commonly employed in such devices.
One method proposed for establishing a bond between a bump or sphere and the semiconductor contact pad is to ball-bond a wire to the pad in a manner similar to wire bonding techniques, and then sever the wire above the ball by weakening the wire either through mechanical manipulation or through formation of a coarse crystalline structure above the ball. (See U.S. Pat. No. 4,442,967 issued to van de Pas, which is incorporated by reference herein.) While generally adequate for providing a strong bond, such a technique presents some difficulties in reliably severing the wire.
It is therefore an object of the invention to provide strong bonds between contact portions and semiconductor chip bonding pads in a reliable manner so that conductors can be bonded to the contact portions to provide external connection to the chip.